Photographic technology has enjoyed an ever increasing growth since its inception more that 150 years ago. What began as a curiosity which required long exposure times to produce low quality image reproductions has advanced to become a large and diverse technology having widespread utility for both commercial and recreational purposes.
Photography has, until recently, been a process based upon chemical reactions, typically of photosensitive salts of precious metals such as silver and platinum. While such processes have been optimized to provide high sensitivity, good resolution and reliable performance, they suffer from several shortcomings. Chemical based photographic systems consume relatively large amounts of both precious metals and specially synthesized organic chemicals; consequently chemical processes tend to be fairly expensive. Furthermore, chemical processes require fairly strict control of time and temperature conditions in order to produce uniformly reliable results. Additionally, chemical based photographic systems require the storage and deployment of relatively large amounts of photographic film within a camera, and necessitate, in most instances, complex processing equipment.
Because of the foregoing limitations of chemical photographic systems, the industry has explored the possibility of adapting presently emerging electronic imaging technologies to photographic systems. Image scanners are enjoying growing utility in a variety of products and for a diversity of applications such as television cameras, input of alpha numeric data to computers and the like and machine vision systems. Optical scanning systems typically include one or more photosensor arrays, each array including a plurality of photoresponsive elements. For purposes of describing the present invention the term "photoresponsive element" shall be broadly applied to include any element capable of producing a detectable electronic signal in response to the absorption of illumination incident thereupon. By way of example, such detectable signals may be provided by a detectable change in voltage, current, resistivity, capacitance or the like.
Electronic photosensor arrays are capable of providing a signal corresponding to a pattern of information projected thereupon an consequently may be utilized in place of conventional photographic film to provide an electronic imaging system free of the shortcomings inherent in chemical based photographic systems.
Charge Coupled Devices (CCDs) are one type of photosensor array which have heretofore been employed in electronic photographic applications. CCDs are solid state devices, typically formed from single crystal silicon and which include therein an array of photoresponsive elements. CCDs have a high degree of photosensitivity and are capable of providing high resolution images. However, CCDs are relatively small in size; the typical CCD array is a two dimensional matrix approximately one centimeter square, and the largest CCDs currently produced are one dimensional arrays no greater than approximately 3 to 4 inches in length. These size constraints impose restrictions on the utility of CCDs for electronic photographic applications. While CCDs generally provide fairly high resolution, the fact that in a camera having practical utility, an optical system must be utilized to project a reduced size image of the object being photographed onto the surface of the CCD effectively reduce the resolution of the CCD.
The optical system itself degrades image resolution somewhat, but, the actual reduction process is the factor which most severely degrades the effective resolution of the image formed by a CCD. For example, a typical two dimensional CCD array is one centimeter square and includes therein 256,000 photosensor units, generally referred to as pixels. To translate this into photographic terminology, the equivalent resolution would be about 50 lines/mm for the one centimeter square CCD array. When an image or other pattern of information occupying an area of 35.times.35 millimeters is projected onto this one centimeter square charge coupled device, the effective resolution of the 35 millimeter square image is reduced to approximately 15 lines/mm. For the sake of comparison, medium resolution photographic film is generally capable of resolving approximately 120 lines/mm. Efforts to improve resolution using single crystal integrated circuits, such as CCDs, encounter at least two significant problems. The first is that integrated circuit chips formed on single crystal silicon wafers must be as small as possible to provide acceptable yields and to meet requirements of economical manufacturing. The second problem, which is intimately related to the first, is that in order to increase the packing density of elements in the small available chip area, finer and finer photolithography must be used with resulting increases in the cost of manufacture. For these reasons, among others, high resolution electronic photography (high resolution being defined relative to chemical photographic capabilities) utilizing present CCD technology is not economically achievable. A direct analogy would be that employing conventional CCD technology in a camera is akin to taking photographs on high grain (50 lines/mm) photographic film, utilizing a format which provides negatives 1 centimeter square. It is simply not possible to obtain good quality enlargements from such a combination.
With improvements in lithographic techniques, it is anticipated that one centimeter square CCDs may ultimately be fabricated to include 1.4 million pixels therein. This translates to a resolution of approximately 120 lines/mm on the one centimeter square device and a corresponding effective resolution of 34 lines/mm for a 35 millimeter square pattern of information projected thereonto. The only way the resolution of the CCD could be further increased is by increasing either the density of pixels in the CCD or the size of the device itself. Both approaches present significant problems. On one hand, the diffraction limit of light will ultimately impose limits on any photolithographic process utilized to pattern CCDs, although constraints of practicality and cost will generally intervene first to set an economic limit on pixel density. On the other hand, processing constraints will limit the size of crystalline CCDs that can be manufactured. Single crystal wafers cannot generally be economically manufactured in sizes exceeding perhaps six to eight inches in diameter. Furthermore, processing steps can introduce defects into such devices.
Increasing the size of a crystalline device, especially while maintaining strict limits on the size of the photolithographic features thereof imposes a great burden of cost insofar as the likelihood of creating defects exponentially increases along with a dramatic decrease in the yield of devices. The result is that the cost of the finished product increases exponentially with increasing device size. It will thus be appreciated that even utilizing the most optimistically projected pixel densities and single crystalline CCD sizes, electronic cameras capable of providing reasonably high resolution photographs of a practical size cannot be economically manufactured utilizing such technology.
Deposited thin film devices represent another approach to the fabrication of photosensor arrays for electronic photography. Thin film devices may be economically manufactured over large areas by the vapor deposition of successive layers of appropriately selected semiconductor alloy materials onto a variety of substrates. By patterning these layers, as for example, through the use of presently available photolithographic techniques, a variety of high resolution device configurations may be provided. It is toward the object of obtaining high resolution electronic images utilizing such large area arrays of thin film photosensitive elements that the instant invention is directed.
Recently, considerable progress has been made in developing processes for depositing thin film semiconductor materials. Such materials can be deposited to cover relatively large areas and can be doped to form p-type and n-type semiconductor materials for the production of semiconductor devices such as p-i-n type photodiodes equivalent, and in some cases superior to those produced by their crystalline counterparts. One particularly promising group of thin film materials are the amorphous materials. As usd herein, the term "amorphous" includes all materials or alloys which have long range disorder although they may have short or intermediate range order, or even contain at times, crystalline inclusions. Also as used herein, the term "microcrystalline" is defined as a unique class of said amorphous materials characterized by a volume fraction of crystalline inclusions, said volume fraction of inclusions being greater than a threshold value at which the onset of substantial changes in certain key parameters such as electrical conductivity, band gap and absorption constant occur.
It is now possible to prepare by glow discharge, or other vapor deposition processes, thin film amorphous silicon, germanium or silicon-germanium alloys in large areas, said alloys possessing low concentrations of localized states in the energy gap thereof and high quality electronic properties. Techniques for the preparation of such alloys are fully described in U.S. Pat. Nos. 4,226,898 and 4,217,374 of Stanford R. Ovshinsky, et al., both of which are entitled "Amorphous Semiconductor Equivalent to Crystalline Semiconductors" and in U.S. Pat. Nos. 4,504,518 and 4,517,223 of Stanford R. Ovshinsky, et al., both of which are entitled "Method of Making Amorphous Semiconductor Alloys and Devices Using Microwave Energy"; the disclosures of all of the foregoing patents are incorporated herein by reference